It is the case. Missing and Nothing behave a little differently in those Unions if they are present as missing or nothing in a vector (see below) – but not if they are in the Union type and not present in the vector. That’s why I asked “How is this being determined”?

because if not 5 then 4or6or6ish – my guess:
it has to be < lbits_ina_byte for bitset speed without too much typeish mem overhead (which would be slowing overall). As we do not yet know how best to utilize the possibles, it was deemed smart to hold a bit or two in reserve while the better use comes to the fore. .there is more involved than altering a constant. tbd.